Traditional physiotherapy is often perceived as repetitive and boring by patients, leading to issues in maintaining their interest and performing the required exercises. This paper investigates older patients' engagement and motivation for physical activity using a serious game-based rehabilitation compared to a control group using a standard care therapy, during hospitalization. We conducted a randomized clinical trial with 57 geriatric patients and demonstrated that the serious game-based rehabilitation was as efficient as the standard rehabilitation program in terms of improving their functional capacity. In this paper, we mainly focus on reporting qualitative analysis of the patient's behavior change thorough the intervention. The intervention lasted 3 weeks. Semi-structured interviews, including all the participants, were conducted before and after the intervention. Patients' attitudes towards rehabilitation process, physical activity and innovation technology were investigated. Motivation to be active following the intervention were explored. Participants admitted feeling bored at the hospital and only following along the care process routine. Enthusiasm towards the progress brought by technological solution was observed. At the end of the hospital stay, the serious game-based rehabilitation received positive feedback. Patients felt more active and observed significant improvement of their general condition. The analysis suggests that patients participating in the gamified rehabilitation are more motivated. Several recommendations on designing serious game for elderly rehabilitation are proposed.